1. Field of the Invention
The present invention relates to a fuel supply control device of an engine.
2. Description of the Related Art
Known in the prior art is an internal combustion engine which is provided with a canister for temporarily storing vaporized fuel, has an air-fuel ratio sensor arranged in the engine exhaust passage, and corrects the amount of fuel injection by a feedback correction coefficient so that the air-fuel ratio becomes a target air-fuel ratio. In this internal combustion engine, when the vaporized fuel stored in the canister is not purged inside the engine intake passage, the feedback correction coefficient changes about a reference value, for example, 1.0. Next, when the purge is started, the amount of fuel injection must be reduced by the amount of vaporized fuel purged so as to maintain the air-fuel ratio at the stoichiometric air-fuel ratio, so the feedback correction coefficient becomes smaller, then for a while after that the feedback correction coefficient is maintained at the small value.
In this case, if, for example, it is assumed that the air-fuel ratio fluctuates 20 percent due to the purged vaporized fuel, the amount of fuel injection must be reduced 20 percent, therefore, the feedback correction coefficient becomes 0.8. If, however, the engine is accelerated in this state and, for example, the amount of intake air becomes double, if the amount of fuel vapor purged is the same, the amount of fluctuation of the air-fuel ratio due to the fuel vapor becomes 10 percent and therefore unless the feedback correction coefficient rises to 0.9, the air-fuel ratio cannot be maintained at the stoichiometric air-fuel ratio.
The feedback correction coefficient, however, is determined so as to change relatively slowly by a predetermined integration constant so as to avoid sudden changes in the air-fuel ratio, so it takes time for the feedback correction coefficient to rise from 0.8 to 0.9 and the air-fuel ratio during that period deviates by a large amount to the lean side with respect to the stoichiometric air-fuel ratio. To prevent the air-fuel ratio from deviating by a large amount with respect to the stoichiometric air-fuel ratio, it becomes necessary to maintain the feedback correction coefficient as much as possible near the reference value, that is, 1.0, even during a purge.
There is known an internal combustion engine (see Japanese Unexamined Patent Publication No. 2-19631) wherein it is attempted to return the feedback correction coefficient to the reference value at the same time as reducing the amount of fuel injection by the amount of reduction of the feedback correction coefficient when a purge is performed and the feedback correction coefficient becomes small.
Even if the feedback correction coefficient is returned to the reference value in this way, however, if the engine is accelerated during the purge action, the air-fuel ratio fluctuates considerably. That is, if the opening of the purge control valve is constant, the amount of purge decreases the smaller the negative pressure in the intake air passage. Therefore, the less the concentration of the purge vapor in the intake air and the more the increase air, the less the concentration of the purge vapor in the intake air. Therefore, at times like acceleration, the negative pressure in the intake passage becomes smaller and further when the amount of intake air increases, the concentration of the purge vapor in the intake air decreases considerably.
Therefore, if there is acceleration during the purge, even if the feedback correction coefficient is returned to the reference value such as in the above-mentioned internal combustion engine, the concentration of the purge vapor in the intake air drops considerably, so the problem arises that the air-fuel ratio becomes lean.